Siemens Simatic S7-300 CP 341 User Manual page 57

Fetching data
The FETCH request is executed in the following sequence:
● Active partner
Sends a FETCH message frame. This contains the message frame header.
● Passive partner
Receives the message frame, checks the header, fetches the data from the CPU, and
acknowledges this with a response message frame. This contains the data.
● Active partner
Receives the response message frame.
If the volume of user data exceeds 128 Bytes, the active partner sends a continuation
FETCH message frame. This contains byte 1 to 4 of the message frame header.
● Passive partner
Receives the continuation FETCH message frame, checks the header, fetches the data
from the CPU, and acknowledges this with a continuation response message frame
containing further data.
If there is an error number (not equal to 0) in the 4th byte, the response message frame does
not contain any data.
If more than 128 bytes are requested, the extra bytes are automatically fetched in one or
more continuation message frames.
Note
If the CPU does not receive the FETCH message frame error-free or if an error has occurred
in the message frame header, the communication partner enters an error number in the 4th
byte of the response message frame. This does not apply when protocol errors occur.
CP 341 Point-to-Point Communication, Installation and Parameter Assignment
Manual, 09/2008, A5E02191071-01
www.gkbpx.com
Basic Principles of Serial Data Transmission
2.5 Data Transmission with the RK 512 Computer Connection
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